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The topmost layer on the surface is the soil (other than the exposed rocks)
which plays an important role in groundwater recharge from rainfall. snowfall
etc. Soils are classified according to the particle size as shown in the table :
Table 6.2 : Soil classification based on particle size (Todd, 2004)
Material
Particle size, mm
Clay
0.004
Silt
0.004 - 0.062
Very fine sand
0.062 - 0.125
Medium sand
0.25 - 0.50
Coarse sand
0.50 - 1.00
Very coarse sand
1.0 - 2.0
Very fine gravel
2.0 - 4.0
Medium gravel
4.0 - 8.0
Coarse gravel
16.0 - 32.0
Very coarse gravel
32.0 - 64.0
Grain size analysis can be performed by mechanical analysis of sieves having
different mesh sizes. ISI standard sieves in the following mesh sizes are
generally adequate for groundwater studies :
25, 20, 16, 10, 6.3, 4, 2, 1 milli metre and
710, 600, 425, 300, 125, 90, 75, 63 and 45 microns (
1
/1000 mm).
Results of the sieve analysis are plotted in a cumulative curve as shown in
Figure 6.3. From the cumulative curve the following analysis can be performed
which are of importance to groundwater analysis :
1
st
Quartile (d75) : Size of sieve which separates 25 percent of coarser
materials from the rest.
Median (d 50) : Size of sieve which separates the materials into two equal halves.
